AGAIN, DO NOT UPDATE 4.3 VIA OTA OR ODIN IF YOU DON'T WANT TO VOID YOUR WARRANTY. IF YOU HAVE NO PROBLEM WITH THAT, FEEL FREE TO UPDATE
Info on KNOX:
What we know so far (I think!)
1. Upgrading to newer Samsung firmware will upgrade the bootloader to knox bootloader. this will give an additional 2 lines in download mode about knox status.
2. If the Knox warranty void line says 0x1 then you cannot use knox softwareas your device has been flagged as insecure. By this i mean that if your workplace / company supports bring your own device to work for corporate emails etc and they use knox to keep security your device will not allow this. You are still able to use future samsung firmware releases with the knox flag 0x1. hope this makes sense. sorry for any confusion this has caused!
3. If the knox warranty line says 0x0 knox will work.
4. with the new knox bootloader root will work, however rooting with CFautorootwill trip the knox flag. thanks @DjeMBeY for correcting this point you can use the root method (Root de la Vega) described here for rooting a device with knox bootloader.
5. If you try to downgrade to earlier firmware / bootloader it will set the knox flag to 0x1 i.e. void.
6. once the knox flag gets set to 0x1 there is no way to set it back (that anyone has found yet anyway!)
7. some newer 4.2.2 software that's supplied with the knox bootloader has been repacked by devs to not use the new bootloader and it all works fine you can still keep up to date.
I've seen people trying the test build of 4.3 touchwiz without the new knox bootloader sound and wifi does not work. This is no longer the case, see point 12.
9. There seems to be a big question mark as to weather Samsung will allow hardware repairs under warranty with the knox 0x1 flag
10. only seems to affect GT-i9505 at the moment!
11. Software affected thanks to @DjeMBeY
12. it is now possible to install custom 4.3 TW roms which have been modified to work without the new knox bootloader, do make sure that they are free from knox bootloader before flashing. Do note also that if you already have knox bootloader flashing anything even these 4.3 custom roms will still trip your 0x1 flag
Thanks to @Mr Impossible @Jamison904 @jovy23 and everyone else!